Father diagnosed with Alzheimer's dementia

Our father is 88 years old and my brother, sister and I love him and want what's best for him. None of us live near Dad, and we each offered for him to live with or near us, but he chose to stay in Michigan and lives in an assisted living facility. Dad used to keep us informed on his legal and medical issues, but that has changed. We have observed forgetfulness, confusion, inappropriate anger, and lack of empathy towards us.

Dad has a married female friend “Sue” that takes him to doctor’s visits, other appointments, and runs errands because she likes to help old people. Sue visits Dad frequently and is the only person the assisted living facility will notify when there are issues. Within the past year, Dad completed new legal documents naming Sue as Power of Attorney, Healthcare POA, and Personal Representative/Executor of his will. As Dad's eldest child, I previously had POA and executor duties. Dad also made Sue joint owner of a checking account where the majority of his retirement income is deposited. 

Last month, Sue told us Dad was diagnosed with late onset Alzheimer’s dementia, mild to moderate stage. She does not normally keep us updated on Dad’s medical appointments, though we have asked. Only Sue has all details on Dad’s finances, and she refused my offer to take them over after she vented about all the things she does for Dad. Neither Dad nor Sue are willing provide us with a copy of his current legal documents. My siblings and I do not know Sue well and have concerns about Dad’s decisions.

Has anyone have ideas on what to do in this situation?